The objective of this project is to examine a fundamental motor control problem: the coordination of muscle activity during the transition from posture to movement. This will be addressed through two types of studies: (1) an experimental study of how neural control strategies are changed as a joint becomes free to move, and (2) a "virtual arm" study of the role actual dynamic forces play in trajectory planning. The first Aim is to examine how the transition from low to high impedance of the environment (while keeping required joint torque constant) affects the neural control mechanisms. In isometric tasks, no motion is allowed and the environment is stiff (by definition). During movement, the environment must be compliant in order for motion to occur. It is this transition that will be explored. It is hypothesized that as the environmental impedance becomes less than infinite (i.e., the potential for movement begins), different muscle coactivation relationships will emerge. Furthermore, it is believed that the relative levels of activation for various muscles will change as external impedance is altered. In the second Aim, the transition from posture to movement will be explored by altering the components of the neuromuscular system in a virtual environment. In this way the control of movement can be studied without having any movement take place. This approach will allow one to study the roles of individual muscles in trajectory formation and the changes in trajectory formation associated with alterations in afferent feedback and muscle force generation capacity. Subjects will be instructed to produce virtual trajectories by moving a simulation of their arm on a computer monitor to specified points, driving it with their own EMG signals. Their actual arm, however, will not be allowed to move. Hence, the neural control system will be forced to produce dynamic activation profiles, while held in a static posture. It is hypothesized that virtual trajectories will show similar muscle activation patterns and the same bell-shaped velocity profiles observed in normal movements. Furthermore, it is hypothesized that by changing the maximum muscle force in the virtual arm for one muscle (e.g., making it theoretically twice as powerful or half as powerful), the CNS will adapt a new control strategy. Exploration of the strategies used by the nervous system to control limb postures and movements lies at the foundation of motor control physiology. This work is potentially beneficial to the understanding of motor control disorders such as spasticity resulting from cerebral palsy, Parkinson's disease, or stroke. The result of these disorders can be explained in terms of inappropriate control strategies. Furthermore, the virtual arm provides a theoretical platform for studies of functional neuromuscular stimulation as well as myoelectric control of prosthetic limbs.